The invention relates to a press/heating platen for continuously operating presses for the production of particle boards, fiber boards or similar wood boards and plastic boards.
Various designs of press/heating platens are used in continuously operating presses according to German Offenlegungsschrift/Patent Specification 21 57 746, 25 45 366, 31 33 817, 39 14 105, 40 17 791 and German Utility Model 75 25 935.
In the prior art, in order to control the procedure, all continuously operating presses must accurately reproduce the process sequence, as known from the intermittent-operation press technology for the production of particle boards, MDF (Medium Density Fiber) boards or OS (Oriented Strand) boards. The procedure for the pressing force action and degasification time upon the pressing stock is essentially executed by longitudinal deformation along the pressing zone. In continuously operating presses, the spherical deformation occurring transversely to the pressing zone is additionally necessary. Therefore, at least one of the two press/heating platens must be deformable between flatness (primarily in the calibration area in the exit region of the pressing zone, the low-pressure area) and a convex geometry (primarily in the entry region of the pressing zone, high-pressure and central medium-pressure area). The amount of deformation also depends upon the board thickness of the pressing stock, the bulk density and the moisture content of the particle/fiber/pressing stock. It is also important to be able to set the minimum possible pressing factor (the maximum possible steel-belt production speed) where optimal physical process requirements are placed upon the pressing stock itself, such as transverse tensile strength and bending strength. It is necessary along the pressing zone, longitudinally and transversely, to be able to set different nip clearances between the upper and lower press/heating platen, and be precise with the following nip clearance differences: longitudinal deformability (Delta I) about 0 to 3 millimeters per meter and transverse deformation (Delta q) about 0 to 1 millimeter per meter.
According to the continuously operating presses which are currently available, the press/heating platens used are deformed either one-dimensionally or two-dimensionally. The continuously operating presses hitherto constructed by the applicant according to German Offenlegungsschrift 40 17 791 operate on the principle of a top-piston press, according to which the deformability of the press/heating platens is brought about in the following manner. The longitudinal deformation is effected one-dimensionally at the upper press/heating platen, which is connected non-positively to the flexibly controllable press beam system and controlled by means of the lateral, upper press pistons. The transverse deformation is effected two-dimensionally with the lower press/heating platen by means of the multi-pot short-stroke cylinders.
According to the continuously operating press disclosed in German Patent Specification 31 33 817 and German Patent Specification 39 14 105, the spherical action upon the pressing stock is effected two-dimensionally with the upper press/heating platen. The longitudinal deformation is effected by the hydraulic actuator rows in each press frame along the pressing zone. The deformability is systematically restricted, due to the slab (having a counter-heating system), and by the higher inherent stiffness. The transverse deformation is herein effected essentially by the counter-heating in the slab. The counter-heating temperatures can be set differently along the pressing zone; e.g., in the front region, in order to give a stronger convex deformation, lower counter-heating temperatures are employed relative to the press/heating platen temperature. By contrast, in the exiting low-pressure region of the pressing zone, in order to obtain the necessary flatness tendency, the counter-heating temperatures are set higher. The convex bending deformation is additionally supported at each press frame by minimizing the hydraulic actuating forces of the outer cylinders of the actuator row. However, this is only possible to a limited degree, since the slab system possesses high inherent stiffness. By virtue of the fact that the entire system operates relatively sluggishly in terms of controllability transverse to the direction of transport of the pressing stock, an on-line adjustment in economic fashion is not possible in just a few seconds.
In continuously operating presses having bottom-piston systems, as in German Offenlegungsschrift 21 57 746, German Offenlegungsschrift 25 45 366 and German Utility Model 75 25 935, the lower press/heating platen, as in the previously described top-piston system, is deformed two-dimensionally. The press/heating platen is of relatively thin and consequently very flexible construction. The lesser press/heating platen thickness dictates very narrow support spacings of the cylinder-piston arrangements. This design however, as a result of the multiple cylinder-piston arrangements and the increased number of actuator rows, is complex in production engineering terms and also very expensive.
The drawbacks of the previous top and bottom-piston systems and the resulting problems they experience are as follows.
The two-dimensional deformation of a press/heating platen has, for a given press/heating platen thickness, a lesser flexibility relative to the one-dimensional deformation. In accordance with the previously mentioned requirement profile, this flexibility is about a third less, due to the additional buckling stresses which are generated during the two-dimensional deformation.
In order to get around this drawback, a dual-function system, an expensive system in design terms, such as the top-piston press according to German Offenlegungsschrift 40 17 791, can be used. A higher flexibility in respect of a two-dimensional deformation can be achieved, as in the continuously operating presses described in German Offenlegungsschrift/Patent Specification 21 57 746, 25 45 366, 31 33 817, 39 14 105 and German Utility Model 75 25 935, with very small support spacings, since the heating/press platen can be designed to be relatively thin and hence highly flexible. However, apart from the additional complexity in machine construction, a further expense is incurred for the additional control requirement of the large number of hydraulic actuators.
Larger support spacings, for example along the pressing zone, lead to thicker heating/press platens. Otherwise, thinner heating platens again have to be used as a result of other design concerns, for instance a supporting slab having counter-heating, thereby giving rise, once again, to the described production-engineering drawbacks.